A plasma supply device is a plasma power supply that supplies plasma processes with power. The plasma supply device operates at a basic frequency that, when used as a plasma power supply, should only deviate slightly from a theoretical value. Typical basic frequencies are, for example, 3.39 MHz, 13.56 MHz, 27 MHz, 40 MHz, and 62 MHz. The inverter, which has at least one switching element, generates from the DC signal of a DC power supply an alternating signal that changes its sign periodically at the rate of the basic frequency. For this purpose, a switching element is switched backwards and forwards between a conducting and a non-conducting state within the cycle of the basic frequency. An output network generates from the alternating signal generated by the inverter a sinusoidal output signal at essentially the predetermined basic frequency.
A plasma is a special aggregate condition that is produced from a gas. Every gas consists in principle of atoms and/or molecules. In the case of a plasma, the gas is largely ionized, which means that the atoms and/or molecules are split into positive and negative charge carriers, i.e., into ions and electrons, due to the supply of energy. A plasma is suitable for machining workpieces because the electrically charged particles are highly reactive chemically and can also be influenced by electrical fields. The charged particles can be accelerated by means of an electrical field on a workpiece, where they can release individual atoms from the workpiece on collision. The released atoms can be removed by gas flow (etching) or coated on other workpieces (production of thin films). A plasma can be used to machine extremely thin layers, for example, in the region of few atom layers. Typical applications for plasma machining are semiconductor technology (coating, etching, etc.), flat screens (similar to semiconductor technology), solar cells (similar to semiconductor technology), architectural glass coating (heat protection, dazzling protection, etc.), storage media (CD, DVD, hard discs), decorative coatings (coloured glasses, etc.), and tool hardening. These applications impose high demands in terms of accuracy and process stability.
To generate a plasma from a gas, energy is supplied to the gas. Energy can be generated in different ways, for example, with light, heat, or electrical energy. If energy is generated using electrical energy, then the plasma is ignited with the electrical energy. A plasma for machining workpieces is typically ignited in a plasma chamber, for which purpose an inert gas, e.g., argon, is generally conducted into the plasma chamber at low pressure. The gas is exposed to an electrical field that is produced by electrodes and/or antennae.
A plasma is generated or is ignited when several conditions are met. A small number of free charge carriers must be present, and in most cases, use is made of the free electrons that are always present to a small extent. The free charge carriers are accelerated so much by the electrical field that they release additional electrons when colliding with atoms or molecules of the inert gas, thus producing positively charged ions and even more negatively charged particles (electrons). The additional free charge carriers are again accelerated and on collision produce additional ions and electrons. An avalanche effect is created. The natural recombination counteracts the constant generation of ions and electrons, i.e., electrons are attracted by ions and recombine to form electrically neutral atoms and/or molecules. Therefore energy is constantly supplied to an ignited plasma in order to maintain it.
Plasma power supplies are used for generating or igniting and maintaining a plasma, but can also be used for exciting gas lasers. Plasma power supplies have the smaller dimensions to ensure that they can be arranged in the application close to the plasma discharges. They should have the highest possible repeat accuracy and operate precisely, with the smallest possible losses to achieve high efficiency. Further requirements are minimal production costs and high maintenance friendliness. If possible, plasma power supplies are provided without mechanically driven components, and fans can be undesirable because of their limited life and the risk of contamination. Furthermore, plasma power supplies should be as reliable as possible, should not overheat, and should have a long operating time.
Due to the high dynamics and often chaotic conditions in plasma processes, a plasma power supply is subject to much more stringent requirements than any other power supply. An un-ignited gas, which has only a very small number of free charge carriers, has an almost infinitely high impedance. Because of its large number of free charge carriers, a plasma has a relatively low impedance. When the plasma is ignited, therefore, there is a rapid impedance change. Another characteristic of an ignited plasma is that the impedance can vary very quickly and often unpredictably, and the impedance is then said to be dynamic. The impedance of the plasma is still non-linear to a great extent, which means that a variation in the voltage on the plasma does not correlate to a similar variation in current. For example, the current can increase much more quickly as the voltage increases due, for example, to an avalanche effect, or the current can also decrease as the voltage increases at so-called negative impedance.
If a power supply discharges a power in the load direction, such as a plasma load, which flows at finite speed towards the load, but cannot be absorbed there because the same current is not set when the voltage is present on the load due to the different impedance, only that proportion of the power that is calculated from voltage and current to obtain the load is absorbed, the remaining proportion of the power being reflected. In fact this also takes place in power supplies with low frequencies, and also in direct current, but only in the latter does it take place so quickly that the voltage at the output of the power supply has in practice not yet changed by the time the reflected energy returns. To the observer, therefore, this happens simultaneously. However, in high frequency technology with frequencies above around 1 MHz, the voltage and current at the output of the power supply have generally already changed by the time the reflected power returns.
The reflected power has a considerable influence on the power supplies in high frequency technology. Reflected power can destabilize power supplies and prevent the supply systems from operating according to the regulations. Because of incorrect adaptations, the reflected power only has proportions of the basic frequency at constant impedances. The reflected power cannot be blocked or absorbed with filters because filters cannot distinguish between forward (to the load) running waves and backwards (from the load) running waves, and would consequently also block or absorb the forward running waves. In order to reduce or minimize the reflected power, so-called impedance adapter elements or networks are used. Impedance adapter elements or networks can be produced using high frequency technology by combinations of inductances, capacitances, and resistances, with resistances not being absolutely necessary. However, if the load is not a constant impedance, but is a dynamic and non-linear impedance, at least two additional problematic phenomena can arise. First, energies can be generated by the non-linear, dynamic impedance at frequencies that differ from the basic frequency, and proportions of these frequencies are conducted in the direction of the power supply. These are blocked or absorbed by filters. Second, the impedance adapter elements cannot follow the fast dynamic impedance variations sufficiently quickly, thus giving rise increasingly to reflections at the basic frequency, which reflections are conducted by the dynamic impedance to the power supply.
Unlike in other power supply systems, plasma power supplies need to be able to be loaded with any incorrect termination, from no load through short-circuit, from infinitely high capacitive load to infinitely high inductive load. At any point on the Smith graph, a plasma power supply must be able to supply power for at least a short period of time and must not suffer permanent damage in doing so. This is linked to the high dynamics and the often chaotic conditions in a plasma process. In addition, frequencies within a wide range and differing from the basic frequency can occur, and these frequencies should be prevented from causing permanent damage to the plasma power supply. The detection and rapid disconnection of an incorrect terminal are allowed in this case, but the plasma power supply should not be damaged if at all possible.